(1) Field of the Invention
The present invention relates to an image reading apparatus, and particularly to an image reading apparatus that reads an image from a document sheet while carrying the document sheet over a scanner unit thereof.
(2) Description of the Related Art
A method called the “sheet-through method” has been conventionally used by image reading apparatuses, which are generally provided in, for example, scanners, MFPs (Multi Function Peripherals), copy machines and fax machines. Unlike the “platen-set method”, which carries a scanner unit along a document sheet fixed at a scanning area, the “sheet-through method” carries a document sheet over the scanner unit fixed at the scanning area to scan an image from the document sheet.
By the way, if a foreign object such as dirt and dust is attached to part of the scanning area, the scanner unit might scan the foreign object as well when scanning the document sheet. This results in a noise on the scanned image. In the case of the “platen-set method”, the scanner unit moves away from the foreign object as scanning the document sheet. Accordingly, in many cases, the foreign object causes only a small noise. However, in the case of the “sheet-through method”, the scanner unit keeps scanning the foreign object while the document sheet is being conveyed, because the scanner unit does not move. As a result, this sometimes causes a large noise called “a line-shaped noise”.
Usually, the scanner unit includes a CCD or a CMOS, which has a plurality of elements consisted of RGB channels for reading RGB color components respectively. For example, a foreign object attached to the G channel causes a green line-shaped noise extending in the feeding direction of the document sheet.
To solve this problem, techniques for detecting and removing a series of pixels that is appear to be line-shaped noises have been invented (For example, see Japanese laid-open Patent Application Publications No. 2003-8846, No. 2004-297302, No. 2005-124075 and No. 2007-129460). According to these techniques, the RGB color components of a detected noise are corrected according to the RGB color components of pixels surrounding the noise (hereinafter called “the surrounding pixels”).
However, in these techniques, if a table or the like is illustrated on a document sheet, a straight line extending in the feeding direction of the document sheet is read, and the line is misdetected as a line-shaped noise. As a result, the line will be corrected based on the pixels in the surrounding pixels, and the original image on the document sheet might be destroyed.